Chlorocarbons and chlorofluorocarbons have obtained widespread use in recent years as specialty cleaning solvents for applications which include metal cleaning and degreasing and cleaning of printed circuit boards and instruments. Materials which are commonly used as solvents in these applications include 1,1,2-trichloro-1,2,2-trifluoroethane, trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane and an azeotrope of methylene chloride with 1,1,2-trichloro-1,2,2-trifluoroethane. For certain cleaning purposes, the 1,1,2-trichloro-1,2,2-trifluoroethane alone may have insufficient cleaning power while the chlorocarbon solvents may be too aggressive. The azeotrope of methylene chloride and 1,1,2-trichloro-1,2,2-trifluoroethane represents an ideal compromise as far as solvent power is concerned. However, increasing concerns over methylene chloride toxicity have made this solvent an unpopular choice.
At the same time that chlorocarbon toxicity concerns are increasing, modern industrial cleaning problems are becoming more complex and varied, and the requirements for the cleaning solvents are more stringent. A solvent should be low boiling, nonflammable, of low toxicity, and should exhibit a high solvent power for the residues to be removed without being so strong that it attacks the substrate being cleaned.
Desired boiling, flammability, and solvent power characteristics can often be obtained by using mixtures of solvents. However, mixtures are often unsatisfactory because they fractionate to an undesirable degree during use and recovery, making it difficult to reuse a solvent mixture with unchanged composition.
On the other hand, mixtures which exhibit azeotropic or azeotrope-like characteristics are often used because they exhibit a minimum boiling point and do not fractionate upon boiling. This is desirable because in vapor degreasing and other cleaning operations, such as circuit board cleaning in which these solvents are also useful, redistilled material is usually used. Unless the solvent mixture exhibits a constant boiling point, i.e. is an azeotrope or is azeotrope-like, fractionation will occur and cause a change in the composition of the solvent mixture during use which could result in a mixture with less desirable properties. In the case of circuit board cleaning these less desirable properties could include lower solvency for rosin fluxes, reduced inertness toward electrical components and increased flammability.
A number of fluorocarbon based azeotropic compositions have been discovered which have been tested and in some cases used as solvents for cleaning operations. For example, U.S. Pat. No. 2,999,815 discloses the azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with acetone; U.S. Pat. No. 3,903,009 discloses a ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and ethanol; U.S. Pat. No. 3,573,213 discloses an azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane; U.S. Pat. No. 3,789,006 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and isopropanol; and U.S. Pat. No. 3,728,268 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane with acetone and ethanol; and U.S. Pat. No. 2,999,817 discloses the binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and methylene chloride.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes and this obviously complicates the search for new azeotropic systems which have application in this field. Nevertheless, there is a constant effort in the art to discover new azeotropic or azeotrope-like systems which have desirable solvency characteristics and particularly a greater versatility of solvency power.